Diaphragm for electrical reproduction of sound



Patented June 16, 1936 2,044,608

DIAPHRAGM FOR ELECTRICAL REPRO- DUC'IION OF SOUND Edward F. Hartord, Wilmington, Dcl., assignor to Du Pont Viscoloid Company, Wilmington, Del., a corporation of Delaware No Drawing. Application May 3, 1935, Serial No. 19,673

4 Claims. (Cl. 179-181) This invention relates to magnetic diaphragms prise a continuous body 01 the polymerized methyl for electrical reproduction or sound and, more methacrylate plastic in which are dispersed the particularly, to telephone diaphragms, and the individual comminuted particles oimetal. like, made from a plastic composition having a The magnetic properties of the diaphragm may :3 comminuted magnetic metal incorporated therebe varied in accordance with the selection of the 5 in. l magnetic metal, it being well known that mag- Diaphragms used in acoustical systems, as in netic metals differ considerably among themselves the receivers of telephones or other magnetic diin magnetic strength. The magnetic properties of aphragms for electrical reproduction of sound, the diaphragm may also be varied by regulating and designed to be actuated by changes in a magthe percentage of magnetic metal particles con- 1 netic field, have heretofore susually been made tained in it. a of iron, iron alloys, or other magnetic metals. In u p as form. polymerized methyl Such diaphragms have been subjected to certain et ac y at (11505 P 5 the Stifiness 0f the definite objections. They will rust 11 not suitably o r q isi f r use n l p n v r a 15 protected by a surface coating; they are, subject the like. The addition of finely divided mag- 15 to some distortion through fatigue, with the renetic metal tends to increase this stifi'ness in suit that their sensitivity and their ability to accordance with the usual stiffening effects of transmit faithfully become impaired with age. fillers upon plastic substances. It the stiffness of Furthermore, the necessity for achieving a corthe resulting composition is too great, it may be rect combination of susceptibility to a magnetic reduced by the incorporation of plasticizers. Ob- 20 field and of mechanical stifiness and elasticity, vlously the particular percent of plasticizer, it

' has made it necessary in the manufacture of such any. to b us n y p r i r mp i n r diaphragms to work out a careful balance bediflp a partleular p p will be tween the magnetic properties of the alloy and t rm by eerte-in amount Of testing o its stiffness. tain the optimum efiect. However, thiscan be- 25 An object of the present invention is to proreadily accomplished y those skilled in e a t vide a novel type of diaphragm for telephones, p consideration O Present disclosure and the like, which is not subject to rusting. A The method of forming composition of Po yiurther object is to provide a diaphragm wherein merized methyl methacrylate P a t c a d comthe magnetic properties and the mechanical minuted magnetic e a may be carried out in 30 properties may be varied substantially independdiflerent ways, 88 W as the making of the d aently. A still further object is to provide a methphlegm D P In the following pl s et od of preparing diaphragms of superior acoustical ads of carrying out the present invention are givproperties and permanence by the use of a plastic en, parts being by weight throughout:

composition having a comminuted magnetic Eiwfllple Part8 polymerized methyl metal incorporated ther i methacrylate are ball milled with 39 parts or pui- The above objects are accomplished according Verized iron until a uniform, fine grained p l to the present invention by making the telephone resultsdiaphragm, or the like, from polymerized methyl A telephone receiver diaphragm is formed from 40 methacrylate plastic having incorporated therei this composition by molding in a die a disc of 2" 40' a comminuted' magnetic metal. diameter and 0.031" thickness. The die is heat- Polymerized methyl methacrylate "i a, l ed to a temperature of 150 C. and a pressure of transparent, rigid resin capable of being readily 2500 pounds p Square inch is pp d to ell-cot fabricated into various shapes by molding. It the molding. The temperature or the die is subhas been discovered that a diaphragm, such as a sequently reduced to about 80 C. while pressure disc, made essentially of polymerized methyl is still maintained upon the die. Thereafter the methacrylate and having pulverized iron, or ierdie is opened and the disc removed. The resultrous alloy, or other magnetic metal incorporated ing thin disc is ready without further treatment therein, in which the former serves as a binder for use as a telephone diaphragm. or the like.

and the latter as a filler to yield a composition Example 2.-In this example a diaphragm is analogous to existing molding compounds, has made as in Example 1, except that there is added unexpectedly advantageous properties for use in to the composition 0.5. parts of ethyl palmitatc telephone receivers, and the like. It will be unwhich serves as a mold lubricant and facilitates derstoodthattheflnisheddiaphrazmwfllcomrcmovalotthemoldeddilctmmthcdio.

' telephone diaphragm.

Example 3.-l00 parts of polymerized methyl methacrylate, 50 partsof pulverized iron, and 50 parts of acetone are kneaded together to form a dough which is then worked on mixing rolls to eflect thorough and homogeneous dispersion of the pulverized metal. The dough is then formed into a block upon a grooved plate by application of a temperature of C. and pressure of 500 pounds per square inch of area of the plate. This block is chilled under pressure and removed from the press in which it is formed.

From the block are'then sliced sheets of the thickness required for a telephone diaphragm, proper allowance for shrinkage in thickness during subsequent evaporation of the solvent being made. The sheeting is then hung in the air at a temperature of 45 C. until it ceases to lose weight through evaporation of solvent. The sheeting thus seasoned is straightened and polished by pressing between polished metal plates at a temperature of C. and a pressure of 1000 pounds per square inch. The sheets are cooled under pressure. From the straightened and polished sheets the diaphragm discs are blanked out.

Example 4.-Into the bottom half of a die of suitable dimensions is sprinkled comminuted polymerized methyl methacrylate in quantity sufficient to form about of the thickness of the disc to be molded. There is then sprinkled a thin layer of finely divided iron on top of the layer of polymerized methacrylate. There is then added a sprinkling of comminuted polymerized methyl methacrylate sufiicient to complete the filling. out of the die. .Molding under heat and pressure as in Example 1 yields a disc suitable vfor use as a ample 50 parts of polymerized methyl methacrylate are first introduced into the mold, then 39 parts of finely dividediron, and last 50 parts more of polymerized methyl methacrylate.

Example 5.-A diaphragm is made as in Exampie 1 except that the following composition is employed: I

Parts Polymethyl alphamethacrylate; 100 Dibutyl phthalate 5 Pulverized iron 42 Ethyl palmitate 1 Example 6.Molding powders are made by ball milling separately 100 parts of polymerized methyl methacrylate and 1 part of butyl stearate as a mold lubricant, on the one hand, and 100 In carrying out this ex- Molding proaoaaeoe are merely illustrative and that the present invention broadly comprises a diaphragm for electrical reproduction of sound made of polymerized methyl methacrylate plastic having incorporated therein a comminuted magnetic metal. As

shown in the example, the comminuted metal may be uniformly incorporated throughout the plastic or may be disposed in fairly well defined layers.

. According to the present invention, polymerized methyl methacrylate will form a major constituent of the plastic but it will be understood that plasticizers may be included, if desired, and also various modifying agents as mold lubricants, and the like. Relatively smaller proportions of other polymerized esters of methacrylic acid may be included, if desired.

An advantage of the present invention is that it provides a magnetic diaphragm for electrical reproduction of sound which is non-rusting and in which a desired balance between magnetic properties and mechanical stifiness may be to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims. I claim: I

l. A magnetic diaphragm for electrical reproduction of sound, said diaphragm being made of polymerized methyl methacrylate plastic having incorporated therein a comminuted magnetic metal.

2. A magnetic diaphragm for electrical reproduction of sound, said diaphragm being made of polymerized methyl methacrylate plastic having incorporated therein a comminuted magnetic ferrous metal.

3. A magnetic diaphragm for electrical reproduction of sound comprising a thin disc'having a layer of polymerized methyl methacrylate plastic on each surface thereof with at least one intermediate layer of a comminuted magnetic metal.

' l. A magnetic diaphragm for electrical reproduction of sound, said diaphragm being made of polymerized methyl methacrylate having uniiormly incorporated therein a comminuted ferrous metal.

EDWARD F. HARFORD. 

